1. Field of the Invention
The present invention generally relates to polarization mode dispersion compensation, and more particularly, to a device and a method for automatically compensating for polarization mode dispersion (referred to as PMD hereinafter) that optical signals are subjected to while being transmitted in a high-speed optical transmission system.
2. Description of the Related Art
In a high-speed optical transmission system, PMD occurring in a transmission optical fiber is one of the main factors limiting the bandwidth and transmission distance of optical signals. PMD occurs in an optical fiber as a result of a small residual birefringence that is introduced in the fiber core by asymmetric internal stress and strain due to external forces acting upon the fiber as well as random polarization coupling. PMD causes waveform distortion of an optical signal due to differential group delay or differential time delay occurring between two mutually orthogonal polarization states, which is called the “principal states of polarization” (referred to as PSP hereinafter) of a transmission optical fiber. Each of the two PSP components is transmitted without distortion of its waveform. When a differential time delay that substantially equals the differential time delay between two PSP components induced in optical fiber link, but of opposite sign, is artificially applied to the two PSP components, mutual delay is cancelled out to compensate for signal distortion due to PMD.
Prior arts in which the PMD compensation principle is practiced are explained below.
B. W. Hakki proposed a PMD compensation method that is described in an article entitled “Polarization Mode Dispersion Compensation by Phase Diversity Detection” in IEEE Photonics Technology Letters, Vol 9, pp. 121–123 (1997). This technique determines a differential time delay between two PSP components using mixers and applies a differential time delay that substantially equals the determined differential time delay to a variable delay line to compensate for PMD. In this case, however, the two electric signal powers inputted to the mixers must be identical to each other irrespective of a variation in the optical power intensity of the two PSP components. This requires a separate high-speed automatic gain control (AGC) circuit. Accordingly, expensive accurate high-speed electronics are additionally needed as the bit rate increases so that system construction costs are raised and the system structure becomes complicated.
Another prior art employing the PMD compensation principle is disclosed in U.S. Pat. No. 5,930,414, entitled “Method and Apparatus for Automatic Compensation of First-order Polarization Mode Dispersion”. This technique uses a Mach-Zehnder interferometer type of compensator to integrate the electric spectrum of the photo-detected output of the compensator and monitors the integrated value using a single controller to alternately feedback-control an optical delay line and a polarization transformer so as to compensate for a differential time delay due to PMD.
However, this technique must control the polarization transformer for each delay value given by the optical delay line to change all polarization states to find the maximum value of spectrum integration values, so it takes a long time to perform compensation.
Another prior method is disclosed in U.S. Pat. No. 6,130,766 entitled “Polarization Mode Dispersion Compensation via an Automatic Tracking of a Principal State of Polarization.” In this technique, compensation of PMD is carried out in such a manner that the beam of a transmitter is frequency-modulated, and one output side of a polarization beam splitter monitors second-order harmonics of the interference signal of two PSP components to control a polarization controller, such that the second-order harmonics are minimized to extract only one undistorted component among the two PSP components. However, this technique requires a separate device and operation for frequency modulation at the transmitter, and its structure for signal processing is relatively complicated because it employs a digital signal processing mode.